1. Field of the Invention
The present invention relates to an optical based communications network, and more particularly to an optical modulator that is programmable in terms of the amount of chirp that is imparted on the modulated optical signal from the optical modulator.
Introduction
In a general fiber optical communication system, optical signals are sent along an optical fiber communication line to a desired location. One type of the fiber optical communication system that can handle optical signals of multiple channels through wavelength multiplexing is called a wavelength division multiplexed (WDM) system. Chirp is a typical problem in these type of systems. Chirp is the instantaneous broadening of the wavelength (and hence frequency) of the optical carrier. Depending on the particular systems application, chirp could either be desirable or undesirable. At the commonly used communications wavelength of 1550 nm, the conventional single mode fiber exhibits significant dispersion. An optical pulse that is broadened on account of chirp can interact with the dispersion in the fiber and impair the fidelity of data transmission. In this case, chirp is undesirable. However, a compressed (i.e. negative chirped) pulse can evolve over a length of fiber to restore its normal shape, thereby the enhancing fidelity for data transmission. In this case, negative chirp at the point of signal origination may be desirable.
In External Optical Modulators (EOMs), chirp xcex1 can be set to a positive (+xcex1), negative (xe2x88x92xcex1) or zero (0) value. The amount of chirp required can be link specific. For example, optical links of different lengths or other physical differences between optical links often require different chirp values to counteract the effect of fiber dispersion. Therefore, it is desirable to design EOMs with an adjustable chirp value to provide for dispersion compensation as required in an arbitrary optical link.
2. Discussion of the Related Art
Previously, zero chirp, non-zero adjustable chirp, and fixed non-zero chirp modulators have been respectively demonstrated in U.S. Pat. Nos. 5,074,631, 5,303,079 and 5,408,544, all of which are expressly incorporated by reference, so-called, zero chirp (U.S. Pat. No. 5,074,631) and non-zero adjustable chirp (U.S. Pat. No. 5,303,079) EOMs have been based on a dual signal electrode design. Non-zero fixed chirp modulators (U.S. Pat. No. 5,408,544) have been implemented either in a single input signal electrode or a dual input signal electrode design. However, there are several drawbacks related to the foregoing related art design of modulators with adjustable or set chirp value.
For applications requiring a tunable chirp modulator, the tunable two-electrode design described in U.S. Pat. No. 5,303,079 is cumbersome to implement. In such a tunable chirp design, a second input signal is connected to a second input signal electrode and is typically derived from a signal that is input into a first input signal electrode. Such tunable designs require an increase in drive circuitry needed for operating an optical modulator because they require two signal electrode drivers, and thus cause an increase in device complexity, size and/or cost. Moreover, it is difficult to precisely set the phase and amplitude balance of the drive signals applied to the two input signal electrodes at high grade rates.
FIGS. 1 and 2 illustrate a zero chirp type optical modulator of the related art. FIG. 1 is a plan view of a single input signal electrode related art EOM and FIG. 2 is a cross-section of FIG. 1 taken along line I-Ixe2x80x2. As shown in FIG. 1, the related art EOM includes an optical modulation chip 1 having an electrooptical effect. The optical modulation chip 1 includes a waveguide, such as a Mach-Zehnder Interferometer (MZI), that runs from one to another end of the chip. The waveguide includes a first main channel 8 that branches into separate parallel paths along respective first and second waveguide arms 3, 3xe2x80x2 near one end of the chip 1. Near the other end of the chip 1, the arms 3, 3xe2x80x2 come back together as a second main channel 8xe2x80x2 at the other end of the chip. Directly overlying the first waveguide arm 3 is a first coplanar-strip (CPS) electrode 4 for connecting an input signal. One end of the first CPS electrode 4 is connected to the input signal and the other end of the signal electrode is connected to a termination resistor. Alternatively, both ends of the first CPS electrode 4 can be connected to independent signal sources. Directly overlying the second arm 3xe2x80x2 is a second CPS electrode 5 for connecting to ground. Both ends of the second CPS electrode 5 are connected to ground G.
FIG. 2 shows optical waveguide arms 3, 3xe2x80x2 that correspond to the two arms 3, 3xe2x80x2 of the interferometer shown in FIG. 1. The waveguide arms are regions within an optical modulator chip 1 (e.g., LiNbO3). An insulating buffer layer 2 (e.g., SiO2) is provided on the optical modulator chip 1 between the CPS electrodes 4, 5 and the waveguide arms 3, 3xe2x80x2. The electrode structure of FIGS. 1 and 2 is a CPS electrode structure. In FIG. 2 the dashed lines show a representation of how the electric field lines emanate from the signal CPS electrode 4 and are received by the ground CPS electrode 5 so as to interact with the optical signals as they pass through the optical waveguide arms 3, 3xe2x80x2. The electric field lines shown are not indicative of the actual path that electric field lines would take between the signal and ground CPS electrodes as the electric fields pass through the body of the optical modulator chip 1. However, the electric field lines shown are generally indicative of electric field lines that go through the waveguide arm 3 from the signal CPS electrode 4 and out through waveguide arm 3 to the ground CPS electrode 5.
As shown in FIG. 2, the CPS electrode structure has a symmetric electric field lines that interact with the waveguide branches in a push-pull manner, which results in a modulator output having no chirp. The chirp parameter |xcex1| of an EOM is directly proportional to the asymmetry in the V of the two arms of the interferometer. As depicted in FIG. 2, V is the potential across a cross-section of a waveguide arm. Chirp parameter |xcex1| is defined as a proportion of the V on one arm of the interferometer with respect to the V of the other arm of the interferometer as follows:                               |          α          |                =                  |                                                    V                2                            -                              V                1                                                                    V                2                            +                              V                1                                              |                                    (        1        )            
where V2 is the potential across a waveguide arm 3xe2x80x2 and V1 is the potential across a waveguide arm 3 from the surface of the waveguide arm. The electric fields as shown in FIG. 2 show the chirp xcex1 to be zero because V2 equals V1. It also can be seen from the equation (1) above that when V2 does not equal V1, chirp is present in the modulator output.
FIG. 3 shows one related art approach for causing chirp in an EOM having a single input signal electrode by causing electric field asymmetry (i.e., a structure where V2xe2x89xa0V1) in the CPS structure. This can be accomplished with an asymmetric CPS structure by changing the width of the ground plane. As shown in FIG. 3, the width of the ground plane has been changed by widening the ground CPS electrode 5xe2x80x2. As a result of the change in width of the ground CPS electrode 5xe2x80x2, the electric field lines are more spread out with regard to arm 3xe2x80x2. Therefore, V2 has a smaller value than V1 and results in the modulated optical output of the modulator having chirp.
While the related art modulator of FIG. 3 can be designed with a desired chirp value, the chirp parameter of the modulator is fixed to a single value. In contrast to the above-described dual input signal adjustable chirp modulator, a non-zero fixed chirp modulator including only one input signal electrode cannot be tuned for different values of chirp, and hence is not an attractive design approach for applications where a chirp requirement for a link is unknown. This design constraint would require building a special EOM with a fixed chirp for every conceivable link requirement, resulting in increased costs and impractical design complexity.
It would be desirable to have a single input signal electrode programmable chirp modulator that does not suffer from the drawbacks of the above-described approaches of the related art. Thus, there remains a need in the art for an EOM having programmable chirp in which the value of chirp for an EOM can be changed without the need of a second input signal electrode.
Accordingly, the present invention is directed to an optical modulator that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
In one aspect of the present invention, the chirp of an optical modulator is programmed utilizing only one input signal electrode.
In another aspect of the present invention, the chirp of an optical modulator is programmed utilizing only one ground electrode.
In a further aspect of the present invention, an external optical modulator (EOM) utilizing a single input signal electrode has a plurality of programmable chirp values.
In yet another aspect of the present invention, electric fields are selectively controlled through the arms of an interferometer.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.